System and method for protection during inverter shutdown in distributed power installations

ABSTRACT

A protection method in a distributed power system including of DC power sources and multiple power modules which include inputs coupled to the DC power sources. The power modules include outputs coupled in series with one or more other power modules to form a serial string. An inverter is coupled to the serial string. The inverter converts power input from the string and produces output power. When the inverter stops production of the output power, each of the power modules is shut down and thereby the power input to the inverter is ceased.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation-in-part of U.S. applicationSer. No. 14/323,531, filed Jul. 3, 2014, which is a continuation of U.S.application Ser. No. 12/328,742, filed Dec. 4, 2008 (now U.S. Pat. No.8,816,535 issued Aug. 26, 2014), which is a continuation-in-part of U.S.application Ser. No. 11/950,271, filed Dec. 4, 2007 (now U.S. Pat. No.9,088,178 issued Jul. 21, 2015), which claims the benefit of each ofU.S. Provisional Application Ser. No. 60/916,815, filed May 9, 2007,U.S. Provisional Application Ser. No. 60/908,095, filed Mar. 26, 2007,U.S. Provisional Application No. 60/868,962, filed Dec. 7, 2006, U.S.Provisional Application No. 60/868,851, filed Dec. 6, 2006, and U.S.Provisional Application No. 60/868,893, filed Dec. 6, 2006. The presentapplication also is a continuation-in-part of U.S. application Ser. No.15/369,881, filed Dec. 5, 2016, which is a continuation-in-part of U.S.application Ser. No. 13/372,009, filed Feb. 13, 2012 (now U.S. Pat. No.9,590,526 issued Mar. 7, 2017), which is a continuation of U.S.application Ser. No. 12/329,525, filed Dec. 5, 2008 (now U.S. Pat. No.8,531,055 issued Sep. 10, 2013), which is a continuation-in-part of U.S.application Ser. No. 11/950,271, filed Dec. 4, 2007 (now U.S. Pat. No.9,088,178 issued Jul. 21, 2015). U.S. application Ser. No. 12/329,525,filed Dec. 5, 2008 (now U.S. Pat. No. 8,531,055 issued Sep. 10, 2013)claims the benefit of U.S. Provisional Application Ser. No. 60/992,589,filed Dec. 5, 2007. The present application also is acontinuation-in-part of U.S. application Ser. No. 15/369,881, filed Dec.5, 2016, which is a continuation-in-part of U.S. application Ser. No.14/323,531, filed Jul. 3, 2014, which is a continuation of U.S.application Ser. No. 12/328,742, filed Dec. 4, 2008 (now U.S. Pat. No.8,816,535 issued Aug. 26, 2014), which is a continuation-in-part of U.S.application Ser. No. 11/950,271, filed Dec. 4, 2007 (now U.S. Pat. No.9,088,178 issued Jul. 21, 2015). Each of the above-mentioned disclosuresare included herein by reference in its entirety and for all purposes.For example, embodiments of the various disclosures may be combined inany combination.

The present application claims priority benefits from U.S. provisionalapplication 60/978,764 filed Oct. 10, 2007 by the present inventors, theentire disclosure of which is incorporated herein by reference.

FIELD AND BACKGROUND

The present invention relates to anti-islanding in a distributed powersystem and, more particularly, system and method for protection ofphotovoltaic distributed power equipment and personnel duringanti-islanding.

Utility networks provide an electrical power system to utilitycustomers. The distribution of electric power from utility companies tocustomers utilizes a network of utility lines connected in a grid-likefashion, referred to as an electrical grid. The electrical grid mayconsist of many independent energy sources energizing the grid inaddition to utility companies energizing the grid, with each independentenergy source being referred to as a distributed power (DP) generationsystem. The modern utility network includes the utility power source,consumer loads, and the distributed power generation systems which alsosupply electrical power to the network. The number and types ofdistributed power generation systems is growing rapidly and can includephotovoltaics, wind, hydro, fuel cells, storage systems such as battery,super-conducting flywheel, and capacitor types, and mechanical devicesincluding conventional and variable speed diesel engines, Stirlingengines, gas turbines, and micro-turbines. These distributed powergeneration systems are connected to the utility network such that theyoperate in parallel with the utility power sources.

One common problem faced by modern utility networks is the occurrence ofislanding. Islanding is the condition where a distributed powergeneration system is severed from the utility network, but continues tosupply power to portions of the utility network after the utility powersupply is disconnected from those portions of the network. Allphotovoltaic systems must have anti islanding detection in order tocomply with safety regulations. Otherwise the photovoltaic installationmay shock or electrocute repairmen after the grid is shut down from thephotovoltaic installation generating power as an island downstream. Theisland condition complicates the orderly reconnection of the utilitynetwork and poses a hazard also to equipment. Thus, it is important foran island condition to be detected and eliminated.

Several techniques have been proposed to guard against islanding. Forexample, one method involves the monitoring of auxiliary contacts on allcircuit breakers of the utility system between its main source ofgeneration and DP systems. The auxiliary contacts are monitored for achange of state which represents an open circuit breaker on the utilitysource. The utility circuit breaker is typically monitored and trippedby external protective relays. When a loss of utility is detected by thechange in state of the auxiliary contact of a circuit breaker, atransferred trip scheme is employed to open the interconnection betweenthe utility and the distributed power system. A transferred trip schemeuses the auxiliary contacts of the utility source being monitored. Theauxiliary contacts are connected in parallel with other devices whichcan trigger the trip of the local interconnection breaker. When theauxiliary contacts change state, a trip is induced on the localinterconnection breaker. This prevents an island condition fromoccurring. The drawback of such a method is that often the point ofutility isolation (the point at which the utility circuit breaker opens)is of such a distance from the local distributed power system thatrunning a contact status signal back to the local distributed powersystem control system is not practical.

Anti-islanding schemes presently used or proposed include passiveschemes and active schemes. Passive schemes are based on localmonitoring of the grid signals, such as under or over voltage, under orover frequency, rate of change of frequency, phase jump, or systemharmonics, for example. Active schemes are based on active signalinjection with monitoring of the resulting grid signals, such asimpedance measurement for example, or active signal injection withactive controls, such as active frequency shifting or active voltageshifting for example. With active schemes, some distortion may occur inthe output current waveform, thereby resulting in a tradeoff betweenislanding detection time and waveform distortion, with faster detectiontypically resulting in higher total harmonic distortion.

A conventional installation of a solar distributed power system 10,including multiple solar panels 101, is illustrated in FIG. 1. Since thevoltage provided by each individual solar panel 101 is low, severalpanels 101 are connected in series to form a string 103 of panels 101.For a large installation, when higher current is required, severalstrings 103 may be connected in parallel to form overall system 10. Theinterconnected solar panels 101 are mounted outdoors, and connected to amaximum power point tracking (MPPT) module 107 and then to an inverter104. MPPT 107 is typically implemented as part of inverter 104 as shownin FIG. 1. The harvested power from DC sources 101 is delivered toinverter 104, which converts the direct-current (DC) intoalternating-current (AC) having a desired voltage and frequency, whichis usually 110V or 220V at 60 Hz, or 220V at 50 Hz. The AC current frominverter 104 may then be used for operating electric appliances or fedto the power grid.

As noted above, each solar panel 101 supplies relatively very lowvoltage and current. A problem facing the solar array designer is toproduce a standard AC current at 120V or 220V root-mean-square (RMS)from a combination of the low voltages of the solar panels. The deliveryof high power from a low voltage requires very high currents, whichcause large conduction losses on the order of the second power of thecurrent i². Furthermore, a power inverter, such as inverter 104, whichis used to convert DC current to AC current, is most efficient when itsinput voltage is slightly higher than its output RMS voltage multipliedby the square root of 2. Hence, in many applications, the power sources,such as solar panels 101, are combined in order to reach the correctvoltage or current. A large number of panels 101 are connected into astring 103 and strings 103 are connected in parallel to power inverter104. Panels 101 are connected in series in order to reach the minimalvoltage required for inverter 104. Multiple strings 103 are connected inparallel into an array to supply higher current, so as to enable higherpower output.

FIG. 1B illustrates one serial string 103 of DC sources, e.g., solarpanels 101 a-101 d, connected to MPPT circuit 107 and inverter 104. Thecurrent versus voltage (IV) characteristics is plotted (110 a-110 d) tothe left of each DC source 101. For each DC power source 101, thecurrent decreases as the output voltage increases. At some voltagevalue, the current goes to zero, and in some applications the voltagevalue may assume a negative value, meaning that the source becomes asink. Bypass diodes (not shown) are used to prevent the source frombecoming a sink. The power output of each source 101, which is equal tothe product of current and voltage (P=i*V), varies depending on thevoltage drawn from the source. At a certain current and voltage, closeto the falling off point of the current, the power reaches its maximum.It is desirable to operate a power generating cell at this maximum powerpoint (MPP). The purpose of the MPPT is to find this point and operatethe system at this point so as to draw the maximum power from thesources.

In a typical, conventional solar panel array, different algorithms andtechniques are used to optimize the integrated power output of system 10using MPPT module 107. MPPT module 107 receives the current extractedfrom all of solar panels 101 together and tracks the maximum power pointfor this current to provide the maximum average power such that if morecurrent is extracted, the average voltage from the panels starts todrop, thus lowering the harvested power. MPPT module 107 maintains acurrent that yields the maximum average power from system 10.

However, since power sources 101 a-101 d are connected in series tosingle MPPT 107, MPPT 107 selects a maximum power point which is someaverage of the maximum power points of the individual serially connectedsources 101. In practice, it is very likely that MPPT 107 would operateat an I-V point that is optimum for only a few or none of sources 101.In the example of FIG. 1B, the selected point is the maximum power pointfor source 101 b, but is off the maximum power point for sources 101 a,101 c and 101 d. Consequently, the arrangement is not operated at bestachievable efficiency.

The present applicant has disclosed in co-pending U.S. application Ser.No. 11/950,271 entitled “Distributed Power Harvesting Systems Using DCPower Sources”, the use of an electrical power converter, e.g. DC-to-DCconverter, coupled to the output of each power source, e.g. photovoltaicpanel. The electrical power converter converts input power to outputpower by monitoring and controlling the input power at a maximum powerlevel. This system may be used also to address the anti-islanding issue.

The term “leakage” as used herein refers to electrical power which isradiated or conducted into an electrical signal line typically at lowlevels and typically because of insufficient isolation.

SUMMARY

The following summary of the invention is included in order to provide abasic understanding of some aspects and features of the invention. Thissummary is not an extensive overview of the invention and as such it isnot intended to particularly identify key or critical elements of theinvention or to delineate the scope of the invention. Its sole purposeis to present some concepts of the invention in a simplified form as aprelude to the more detailed description that is presented below.

According to an aspect of the present invention, there is provided in adistributed power system multiple DC power sources and multiple powermodules which include inputs coupled respectively to the DC powersources. The power modules each include outputs coupled in series toform a serial string. An inverter is coupled to the serial string. Theinverter converts power input from the string and produces output power.A protection mechanism in the power modules shuts down the power modulesand ceases the power input to the inverter when the inverter stopsproducing the output power. Typically, the inverter is connected to theelectrical grid. A monitoring mechanism is attached to the electricalgrid which monitors one or more electrical parameters of the electricalgrid. A shutdown mechanism is attached to the monitoring mechanism whichwhen one or more of the electrical parameters is out of predeterminedspecification, the inverter stops the production of the output power ordisconnects from the grid. A switch is preferably disposed between theserial string and the inverter. The switch is activated by the shutdownmechanism and the protection mechanism senses a change in currentflowing through the serial string when the switch is activated. When theswitch is connected serially with the serial string, the protectionmechanism senses that current less than a previously specified minimalthreshold current in the serial string; or when the switch is connectedin parallel with the serial string the protection mechanism senses acurrent greater than a previously specified maximal threshold current inthe string. Alternatively a signal-providing mechanism is attached tothe inverter which provides a signal based on the shutdown mechanism.Multiple receivers are attached respectively to the power modules. Thereceivers receive the signal and multiple enabling mechanisms, which areattached respectively to the receivers, enable the respective powermodules to supply the input power to the inverter based on the presenceof the signal or absence thereof When the signal is a keep-alive signal,the enabling mechanisms enable the respective power modules to supplythe input power to the inverter based on the presence of the keep-alivesignal. When the signal is a shut-down signal, the enabling mechanismdisables the respective power modules and stops supply of the inputpower to the inverter based on the presence of the shut-down signal. Thesignal in the serial string is optionally from the electrical grid anddetected at the frequency of the electrical grid or detected at a higherfrequency up converted from the frequency of the electrical grid. Thesignal in the serial string is optionally from the inverter or theoutput power therefrom, and detected at a switching frequency of theinverter. The signal is optionally superimposed on the power input tothe inverter from the serial string. The signal may be wirelesslytransmitted by the signal-providing mechanism, and the receiver in eachof the power modules, receives the wirelessly transmitted signal.

According to another aspect of the present invention, there is provideda protection method in a distributed power system including DC powersources and multiple power modules each of which include inputs coupledto the DC power sources. The power modules each include outputs coupledin series to form a serial string. An inverter is coupled to the serialstring. The inverter converts power input from the string and producesoutput power. When the inverter stops production of the output power,each of the power modules is shut down and thereby the power input tothe inverter is ceased. When the inverter is connected to and suppliesthe output power to the electrical grid, one or more electricalparameters of the grid are monitored. When the one or more electricalparameters of the grid are out of a predetermined specification, theinverter is shut down and thereby production of the output power isstopped or the inverter is disconnected from the grid. When the inverteris shut down, a switch disposed between the serial string and theinverter is activated. When the switch is activated a change in currentflowing through the serial string is sensed. Alternatively a signal isprovided based on the shutdown mechanism. Multiple receivers areattached respectively to the power modules. The receivers receive thesignals which enable the respective power modules to supply the inputpower to the inverter based on the presence of the signal or absencethereof When the signal is a keep-alive signal, the respective powermodules supply the input power to the inverter based on the presence ofthe keep-alive signal. When the signal is a shut-down signal, therespective power modules stop supply of the input power to the inverterbased on the presence of the shut-down signal. The signal may be basedon current in the serial string from the electrical grid and detected atthe frequency of the electrical grid or detected at a higher frequencyup converted from the frequency of the electrical grid. The signal inthe serial string is optionally from the inverter or the output powertherefrom, and detected at a switching frequency of the inverter. Thesignal is optionally actively superimposed on the power input to theinverter from the serial string. The signal may be wirelesslytransmitted, and the receiver in each of the power modules, receives thewirelessly transmitted signal.

The foregoing and/or other aspects will become apparent from thefollowing detailed description when considered in conjunction with theaccompanying drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, exemplify embodiments of the presentinvention and, together with the description, serve to explain andillustrate principles of the invention. The drawings are intended toillustrate various features of the illustrated embodiments in adiagrammatic manner. The drawings are not intended to depict everyfeature of actual embodiments nor relative dimensions of the depictedelements, and are not necessarily drawn to scale.

The invention is herein described, by way of example only, withreference to the accompanying drawings, wherein:

FIG. 1 illustrates a conventional power harvesting system usingphotovoltaic panels as DC power sources;

FIG. 1B illustrates current versus voltage characteristic curves for oneserial string the DC power sources of FIG. 1;

FIG. 2 illustrates a distributed power harvesting circuit, previouslydisclosed in co-pending U.S. application Ser. No. 11/950,271;

FIG. 3 illustrates an exemplary DC-to-DC converter, previously disclosedin co-pending U.S. application Ser. No. 11/950,271;

FIGS. 4 and 4A illustrate a system for protection during an islandingcondition, in accordance with aspects of the present invention;

FIGS. 4B and 4C illustrate in more detail the system of FIGS. 4 and 4A;

FIG. 4D illustrates a method, according to an aspect of the presentinvention using the system of FIGS. 4 and 4A.

FIGS. 5 and 5A, illustrate a system for protection during an islandingcondition in accordance with other aspects of the present invention;

FIG. 5B illustrates an example wherein a system according to anembodiment of the invention is applied as a retrofit to a prior artsystem, such as the system of FIG. 1.

FIGS. 6, 6A and 6B illustrate a system for protection during anislanding condition, according to still other aspects of the presentinvention; and

FIGS. 7 and 7A, illustrate a system for protection during an islandingcondition, according to yet other aspects of the present invention.

DETAILED DESCRIPTION

Reference will now be made in detail to embodiments of the presentinvention, examples of which are illustrated in the accompanyingdrawings, wherein like reference numerals refer to the like elementsthroughout. The embodiments are described below to explain the presentinvention by referring to the figures.

It should be noted, that although the discussion herein relatesprimarily to anti-islanding in photovoltaic systems and moreparticularly to those systems previously disclosed in U.S. applicationSer. No. 11/950,271, the present invention may, by non-limiting example,alternatively be configured as well using conventional photovoltaicdistributed power systems and other distributed power systems including(but not limited to) wind turbines, hydroturbines, fuel cells, storagesystems such as battery, super-conducting flywheel, and capacitors, andmechanical devices including conventional and variable speed dieselengines, Stirling engines, gas turbines, and micro-turbines.

By way of introduction, it is important to note that aspects of thepresent invention have important safety benefits. While installing orperforming maintenance on photovoltaic systems according to certainaspects of the present invention, installers are protected from dangerof shock or electrocution since systems according to embodiments of thepresent invention do not output high voltage such as when solar panelsare exposed to sunlight. Similarly, firefighters, even after they shutdown the main electrical switch to a burning building can safely breakinto the burning building or hose the roof of the building with waterwithout fear of high voltage DC conduction through the water, since highvoltage direct current feeding the inverter is safely turned off.

Before explaining embodiments of the invention in detail, it is to beunderstood that the invention is not limited in its application to thedetails of design and the arrangement of the components set forth in thefollowing description or illustrated in the drawings. The invention iscapable of other embodiments or of being practiced or carried out invarious ways. Also, it is to be understood that the phraseology andterminology employed herein is for the purpose of description and shouldnot be regarded as limiting.

Referring now to the drawings, FIG. 2 illustrates a distributed powerharvesting circuit 20, previously disclosed in U.S. application Ser. No.11/950,271. Circuit 20 enables connection of multiple distributed powersources, for example solar panels 201 a-201 d, to a single power supply.Series string 203 of solar panels 201 may be coupled to an inverter 204or multiple connected strings 203 of solar panels 201 may be connectedto a single inverter 204. In configuration 20, each solar panel 201a-201 d is connected individually to a separate power conditioner, herea converter circuit or a module 205 a-205 d. Each solar panel 201together with its associated power converter circuit 205 forms a powergenerating element 222. (Only one such power generating element 222 ismarked in FIG. 2.) Each converter 205 a-205 d adapts optimally to thepower characteristics of the connected solar panel 201 a-201 d andtransfers the power efficiently from input to output of converter 205.Converters 205 a-205 d are typically microprocessor controlled switchingconverters, e.g. buck converters, boost converters, buck/boostconverters, flyback or forward converters, etc. The converters 205 a-205d may also contain a number of component converters, for example aserial connection of a buck and a boost converter. Each converter 205a-205 d includes a control loop 221, e.g. MPPT loop that receives afeedback signal, not from the converter's output current or voltage, butrather from the converter's input coming from solar panel 201. The MPPTloop of converter 205 locks the input voltage and current from eachsolar panel 201 a-201 d at its optimal power point, by varying one ormore duty cycles of the switching conversion typically by pulse widthmodulation (PWM) in such a way that maximum power is extracted from eachattached panel 201 a-201 d. The controller of converter 205 dynamicallytracks the maximum power point at the converter input. Feedback loop 221is closed on the input power in order to track maximum input powerrather than closing a feedback loop on the output voltage as performedby conventional DC-to-DC voltage converters.

As a result of having a separate MPPT circuit in each converter 205a-205 d, and consequently for each solar panel 201 a-201 d, each string203 may have a different number or different specification, size and/ormodel of panels 201 a-201 d connected in series. System 20 of FIG. 2continuously performs MPPT on the output of each solar panel 201 a-201 dto react to changes in temperature, solar radiance, shading or otherperformance factors that effect one or more of solar panels 201 a-201 d.As a result, the MPPT circuit within the converters 205 a-205 d harveststhe maximum possible power from each panel 201 a-201 d and transfersthis power as output regardless of the parameters effecting other solarpanels 201 a-201 d.

The outputs of converters 201 a-201 d are series connected into a singleDC output that forms the input to inverter 204. Inverter 204 convertsthe series connected DC output of converters 201 a-201 d into an ACpower supply. Inverter 204, may be set to regulate the voltage at theinput of inverter 204. In this example, an independent control loop 220holds the voltage input to inverter 204 at a set value, say 400 volts.The current at the input of inverter 204 is typically fixed by the poweravailable and generated by photovoltaic panels 201.

In order to legally be allowed to connect to the grid in each country,inverter 104,204 is preferably designed to comply with local electricalregulations. Electrical regulations typically dictate, among otherthings, the minimal and maximal voltages of the grid e.g. 220-260 rootmean squares voltage V, and a range of permitted frequency, e.g 45-55Hz. Whenever the grid deviates from allowed values inverter 104,204 isrequired to disconnect from the grid. Disconnection from the grid istypically performed using software controlling inverter 104, 204 andcontrol circuitry which constantly monitors grid parameters, e.g.voltage, frequency.

In system 10, solar panels 101 are directly connected (e.g. inseries-parallel) to inverter 104. When an islanding condition isdetected, inverter 104 is disconnected from the grid. Hence, inverter104 stops drawing current and therefore panels 101 output a relativelyhigh open circuit voltage typically 25% higher than the normal operatingvoltage. An open circuit voltage 25% higher than nominal working voltageis typically safe, (less than the allowed 600 VDC in the USA and 1000VDC in Europe) which are typical ratings for inverters 104 designed tobe able to handle the higher open circuit voltage.

In system 20, there are power converters 205 which “push” power to theoutput of converters 205. Under an islanding condition which has beendetected by inverter 204, inverter 204 is shut down and current is notflowing between converters 205 and inverter 204. Consequently, in system20, the open circuit voltage at the input to inverter 204, reachesdangerous voltages, higher than the open circuit maximum voltage ratingsof inverters 104, 204.

Reference is now made to FIG. 3 which illustrates an exemplary DC-to-DCconverter 205 previously disclosed in co-pending US application Ser. No.11/950,271. DC-to-DC converters are used to either step down or step upa DC voltage input to a higher or a lower DC voltage output, dependingon the requirements of the output circuit. However, in the embodiment ofFIG. 3 the DC-DC converter 205 is used as a power converter, i.e.,transferring the input power to output power, the input voltage varyingaccording to the MPPT at the input, while the output current is dictatedby the constant input voltage to inverter 104, 204. That is, the inputvoltage and current may vary at any time and the output voltage andcurrent may vary at any time, depending on the operating condition of DCpower sources 201.

Converter 205 is connected to a corresponding DC power source 201 atinput terminals 314 and 316. The converted power of the DC power source201 is output to the circuit through output terminals 310, 312. Betweenthe input terminals 314, 316 and the output terminals 310, 312, theconverter circuit includes input and output capacitors 320, 340,backflow prevention diodes 322, 342 and a power conversion circuitincluding a controller 306 and an inductor 308.

Diode 342 is in series with output 312 with a polarity such that currentdoes not backflow into the converter 205. Diode 322 is coupled betweenthe positive output lead 312 through inductor 308 which acts a short forDC current and the negative input lead 314 with such polarity to preventa current from the output 312 to backflow into solar panel 201.

A potential difference exists between wires 314 and 316 due to theelectron-hole pairs produced in the solar cells of panel 201. Converter205 maintains maximum power output by extracting current from the solarpanel 201 at its peak power point by continuously monitoring the currentand voltage provided by panel 201 and using a maximum power pointtracking algorithm. Controller 306 includes an MPPT circuit or algorithmfor performing the peak power tracking. Peak power tracking and pulsewidth modulation (PWM) are performed together to achieve the desiredinput voltage and current. The MPPT in controller 306 may be anyconventional MPPT, such as, e.g., perturb and observe (P&O), incrementalconductance, etc. However, notably the MPPT is performed on panel 201directly, i.e., at the input to converter 205, rather than at the outputof converter 205. The generated power is then transferred to the outputterminals 310 and 312. The outputs of multiple converters 205 may beconnected in series, such that the positive lead 312 of one converter205 is connected to the negative lead 310 of the next converter 205.

In FIG. 3, converter 205 is shown as a buck plus boost converter. Theterm “buck plus boost” as used herein is a buck converter directlyfollowed by a boost converter as shown in FIG. 3, which may also appearin the literature as “cascaded buck-boost converter”. If the voltage isto be lowered, the boost portion is substantially shorted. If thevoltage is to be raised, the buck portion is substantially shorted. Theterm “buck plus boost” differs from buck/boost topology which is aclassic topology that may be used when voltage is to be raised orlowered, and sometimes appears in the literature as “cascadedbuck-boost”. The efficiency of “buck/boost” topology is inherently lowerthen a buck or a boost. Additionally, for given requirements, abuck-boost converter will need bigger passive components then a buckplus boost converter in order to function. Therefore, the buck plusboost topology of FIG. 3 has a higher efficiency than the buck/boosttopology. However, the circuit of FIG. 3 continuously decides whether itis bucking or boosting. In some situations when the desired outputvoltage is similar to the input voltage, then both the buck and boostportions may be operational.

The controller 306 may include a pulse width modulator, PWM, or adigital pulse width modulator, DPWM, to be used with the buck and boostconverter circuits. Controller 306 controls both the buck converter andthe boost converter and determines whether a buck or a boost operationis to be performed. In some circumstances both the buck and boostportions may operate together. That is, the input voltage and currentare selected independently of the selection of output current andvoltage. Moreover, the selection of either input or output values maychange at any given moment depending on the operation of the DC powersources. Therefore, in the embodiment of FIG. 3, converter 205 isconstructed so that at any given time a selected value of input voltageand current may be up converted or down converted depending on theoutput requirement.

In one implementation, an integrated circuit (IC) 304 may be used thatincorporates some of the functionality of converter 205. IC 304 isoptionally a single ASIC able to withstand harsh temperature extremespresent in outdoor solar installations. ASIC 304 may be designed for ahigh mean time between failures (MTBF) of more than 25 years. However, adiscrete solution using multiple integrated circuits may also be used ina similar manner. In the exemplary embodiment shown in FIG. 3, the buckplus boost portion of the converter 305 is implemented as the IC 304.Practical considerations may lead to other segmentations of the system.For example, in one aspect of the invention, the IC 304 may include twoICs, one analog IC which handles the high currents and voltages in thesystem, and one simple low-voltage digital IC which includes the controllogic. The analog IC may be implemented using power FETs which mayalternatively be implemented in discrete components, FET drivers, A/Ds,and the like. The digital IC may form controller 306.

In the exemplary circuit 205 shown, the buck converter includes inputcapacitor 320, transistors 328 and 330, diode 322 positioned in parallelto transistor 328, and inductor 308. Transistors 328, 330 each have aparasitic body diode 324, 326. The boost converter includes inductor308, which is shared with the buck converter, transistors 348 and 350 adiode 342 positioned in parallel to transistor 350, and output capacitor340. Transistors 348, 350 each have a parasitic body diode 344, 346.

System 20 includes converters 205 which are connected in series andcarry the current from string 203. If a failure in one of the seriallyconnected converters 205 causes an open circuit in failed converter 205,current ceases to flow through the entire string 203 of converters 205,thereby causing system 20 to stop functioning. Aspects of the presentinvention provide a converter circuit 205 in which electrical componentshave one or more bypass routes associated with them that carry thecurrent in case of an electrical component failing within one ofconverters 205. For example, each switching transistor of either thebuck or the boost portion of the converter has its own diode bypass.Also, upon failure of inductor 308, the current bypasses the failedinductor 308 through parasitic diodes 344,346.

Reference is now made to FIG. 4 which illustrates a system 40 forprotection during an islanding condition, in accordance with embodimentsof the present invention. For simplicity, a single string 423 is shownof distributed power sources, e.g solar panels 201 a-201 d connected torespective power converters 405 a-d. Serial string 423 is input toinverter 404 through wires 412 and 410. The output of inverter 404 isconnected to and supplies electrical power to the electrical grid.Inverter 404, typically includes a monitoring, and detection mechanism401 which monitors one or more parameters of the electrical grid such asvoltage and/or frequency. If one or more of the grid parameters is outof specification indicating an islanding condition, monitoring anddetection mechanism 401 typically causes inverter 404 to be shut down orinverter 404 is disconnected from the grid so that output power is nolonger supplied by inverter 404 to the grid. At the same time, a signal414 is transmitted to a switch mechanism 403 which may be located at theinput of inverter 404 before input capacitor 408. Switch mechanism 403is optionally packaged with inverter 404 or may be integrated withinverter 404 and packaged separately. In this example, signal 414activates switch mechanism 403 so that when switch 403 is activated, thecurrent flowing through serial string 423 and wires 410, 412 variesabruptly.

Reference is now also made to FIG. 4A which illustrates in more detailconverter 405. Converter 405 is equipped with a current sensingmechanism 407 which upon sensing a variation in current through serialstring 423 signals controller 306 to shut down and stop convertingpower. Typically, current sensing mechanism 407 includes ananalog/digital converter which continuously feeds data to controller306. Controller 306 detects a shutdown in current and decides to shutdown the converters 405 accordingly.

Reference is now also made to FIGS. 4B and 4C which illustrateschematically switch mechanism 403 in more detail. FIG. 4B illustratesswitch mechanism 403 in a serial configuration in which switch 403 isconnected in series with the serial string 423 and FIG. 4C illustrates aparallel configuration in which switch 403 is connected in parallel withserial string 423. In the serial configuration (FIG. 4B) switch 403 isclosed during normal operation of inverter 404. When an island conditionis detected, serial switch 403 opens during shut down of inverter 404.Current sensing mechanism 407 upon sensing zero current signalscontroller 306 that output current is less than a previously specifiedminimum value and controller 306 shuts down power conversion inconverter 405. In the parallel configuration (FIG. 4C), switch 403 isopen during normal operation of inverter 404. When an island conditionis detected, parallel switch 403 closes during shut down of inverter404. With all the current of serial string 423 flowing through theswitch 403 at minimal load, the current increases to above a previouslyspecified maximum current. Current sensing mechanism 407 upon sensing acurrent maximum signals controller 306 that output current is abovemaximal previously specified value and controller 306 shuts down powerconversion. Switch mechanism 403 in different embodiments may beembodied by a mechanical switch or a solid state switch with current andvoltage ratings appropriate to the present application. Switch mechanism403 is preferably selected by one skilled in the art of powerelectronics so that arcing across its open terminals is avoided whilepracticing some embodiments of the present invention.

Reference is now made FIG. 4D which illustrates a method, according toan embodiment of the present invention. In decision block 450, outputpower from inverter 104, 204 is constantly monitored. If output power isstopped, power converters 405 are shut down.

Reference is now made to FIG. 5, illustrating a system 50 according toother embodiments of the present invention for protection during anislanding condition. For simplicity, a single string 523 is shown ofdistributed power sources, e.g solar panels 201 a-201 d connected torespective power converters 505 a-d. Serial string 523 is input toinverter 504 through wires 412 and 410. The output of inverter 504 isconnected to and supplies electrical power to the electrical grid.Inverter 504, typically includes a monitoring and detection mechanism401 which monitors one or more parameters of the electrical grid such asvoltage and/or frequency. If one or more of the grid parameters is outof specification indicating an islanding condition, monitoring/detectionmechanism 401 typically shuts down inverter 504 or disconnects from thegrid, so that output power is no longer supplied by inverter 504 to thegrid. During normal operation, a line communications transmitter 503superimposes a keep-alive signal, for instance between 1 kilohertz to100 Megahertz on direct current (DC) input lines 410 and 412 attached toserial string 523.

Reference is now also made to FIG. 5A which illustrates converter 505 inmore detail. The keep-alive signal is constantly monitored and detectedby a line communications receiver 507. Only while receiver 507 sensesthe keep-alive signal does receiver 507 provide an enable signal tocontroller 306. When controller 306 doesn't receive an enabling signalfrom receiver 507, controller 306 shuts down power conversion ofconverter 505.

Alternatively, instead of a “keep-alive” signal, a stop signal 514 whichis first generated by monitoring and detection mechanism 401 when anislanding condition is detected, is transmitted to receiver 507. Thestop signal is transmitted over line communications by superimposing avarying (e.g. 10 Khz to 100 Mhz) signal over the power lines of serialstring 523. Receiver 507 receives the stop signal and relays the stopsignal to controller 306 using, e.g., a single disable bit. Controller306 on receiving a disable signal, stops converting power to the outputof converter 505. Typically, when converters 505 are disabled they gointo a bypass mode which allows current from other converters 505 topass through. Hence, the stop signal may be continued until all powerstops being supplied on string 523 by all of converters 505.

It should be noted that one skilled in the art would realize thatalthough in system 50, converters 505 are shown to have feedback loop221, as in controller 205 of system 20, embodiments of the presentinvention as illustrated in system 40 using switch mechanism 403 and/orin system 50 using line communications, to the serial string may beapplied to and find benefit in other distributed power systems usingconverters without feedback loops 221 as applied to prior art system 10.Similarly, conventional inverters 104 may be used instead of inverter504 with communications transmitter 503 added to inverter 104 either bythe inverter manufacturer or as a retrofit. For example, FIG. 5Billustrates a system according to an embodiment of the invention appliedas a retrofit to a prior art system, such as the system of FIG. 1. Inthis example, detection mechanism 401 and switch mechanism 403 areinstalled between the grid and the conventional inverter 104. Of course,detection mechanism 401 and switch mechanism 403 may be incorporatedinto the inverter, e.g., for original installation, rather than aretrofit. Also, other implementations described herein may be usedinstead of detection mechanism 401 and switch mechanism 403. Advantagesof incorporation of monitoring and detection mechanism 401 and one ofswitch mechanism 403 or communications transmitter 503 into system 10 isbeneficial during installation, maintenance, and firefighting.

Reference in now made to FIG. 6 which illustrates system 60, accordingto another embodiment of the present invention for protection during anislanding condition. For simplicity, a single string 623 is shown ofdistributed power sources, e.g solar panels 201 a-201 d connected torespective power converters 605 a-d. Serial string 623 is input toconventional inverter 104 through wires 412 and 410. The output ofinverter 104 is connected to and supplies electrical power to theelectrical grid. Inverter 104, typically includes a monitoring anddetection mechanism 401 which monitors one or more parameters of theelectrical grid such as voltage and/or frequency. If one or more of thegrid parameters is out of specification indicating an islandingcondition, monitoring and detection mechanism 401 typically shuts downinverter 104 so that output power is no longer supplied by inverter 104to the grid. During normal operation, a 100 Hz (or 120 Hz. in USA)ripple current is detectable between lines 410, 412 and in serial string623 since capacitors of inverter 104 do not block entirely thealternating current (AC), or the 100/120 Hz is intentionally leaked intoserial string 623 through lines 410, 412.

Reference is now also made to FIG. 6A which illustrates converter 605 inmore detail. The 100/120 Hz leakage is constantly monitored and detectedby a receiver 607. Only while receiver 607 senses the leakage from thegrid does receiver 607 provide an enable signal to controller 306. Whencontroller 306 doesn't receive an enabling signal from receiver 607,controller 306 shuts down power conversion of converters 605.

Alternatively or in addition, one or more switching frequencies ofinverter 104, typically 16 Khz or 32 KHz. may be detected as leakage orprovided intentionally to serial string 623 along lines 412,410.Receiver 607 is configured to detect the 16/32 KHz inverter switchingfrequency and provides an enabling signal to controller while inverter104 is operating.

Reference is now made to FIG. 6B, showing a simplified block diagramaccording to an embodiment of the present invention for up conversion of100/120 Hz. into a higher frequency in order to enable faster detectionin receiver 607 of leakage from the grid. The 100 Hertz or 100 Hertzsignal is AC coupled by capacitor 631 to remove the direct currentcomponent in serial string 623 and lines 410 and 412. The 100/120 Hz.signal is optionally amplified and rectified by a full wave rectifier635 so that a 100 Hz or 120 Hz unipolar DC ripple is achieved. The100/120 Hz unipolar signal is split. One portion of the 100/120 Hz.unipolar ripple is converted to a square wave, such as in acomparator/digitize circuit 639. A second portion of the 100/120 Hzunipolar ripple undergoes a known phase shift, e.g of 400 Hz. in a phaseshifter 633 and output to a second comparator/digitizing circuit 631.The two outputs of two digitizing circuits 639,631 undergo an exclusiveOR in a XOR circuit 633 which outputs a signal at a much higherfrequency, e.g. 800 Hz.

Reference is now made to FIG. 7, illustrating a system 70 according toother embodiments of the present invention for protection during anislanding condition. For simplicity, a single string 723 is shown ofdistributed power sources, e.g solar panels 201 a-201 d connected torespective power converters 705 a-d. Serial string 723 is input toinverter 704 through wires 412 and 410. The output of inverter 704 isconnected to and supplies electrical power to the electrical grid.Inverter 704, typically includes a monitoring and detection mechanism401 which monitors one or more parameters of the electrical grid such asvoltage and/or frequency. If one or more of the grid parameters is outof specification indicating an islanding condition, monitoring, anddetection mechanism 401 typically shuts down inverter 704 or disconnectsinverter 704 from the grid so that output power is no longer supplied byinverter 704 to the grid. During normal operation, a wirelesstransmitter 703 transmits wirelessly a signal, for instance between 100Megahertz-10 Gigahertz.

Reference is now also made to FIG. 7A which illustrates converter 705 inmore detail. The wireless signal is received and constantly monitored bya wireless receiver 707 Only while receiver 707 senses the wirelesssignal does receiver 707 provide an enable signal to controller 306.When controller 306 doesn't receive an enabling signal from receiver707, controller 306 shuts down power conversion of converter 705.

The present invention has been described in relation to particularexamples, which are intended in all respects to be illustrative ratherthan restrictive. Those skilled in the art will appreciate that manydifferent combinations of hardware, software, and firmware will besuitable for practicing the present invention. Moreover, otherimplementations of the invention will be apparent to those skilled inthe art from consideration of the specification and practice of theinvention disclosed herein. Various aspects and/or components of thedescribed embodiments may be used singly or in any combination in theserver arts. It is intended that the specification and examples beconsidered as exemplary only, with a true scope and spirit of theinvention being indicated by the following claims.

The invention claimed is:
 1. A system comprising: a power-generationsource; a power module comprising a receiver, a controller, inputterminals, and output terminals; an inverter comprising a monitoring anddetection mechanism coupled to an output of the inverter; and asignal-providing device external to the power module, wherein thesignal-providing device comprises a switch serially connected betweenthe inverter and the output terminals of the power module, and whereinthe signal-providing device is configured to: transmit a signal to thepower module using the switch, and open the switch in response to themonitoring and detection mechanism indicating that an electricalparameter sensed at an output of the inverter is out of a predeterminedspecification, wherein: the input terminals of the power module areconfigured to receive input power from the power-generation source, theoutput terminals of the power module are configured to output convertedpower to the inverter, the receiver is configured to receive and monitora signal from the signal-providing device, the controller is configuredto enable and disable the power module according to the signal receivedby the receiver, and the power module is configured to: when enabled,supply the converted power to the inverter, and when disabled, limit theconverted power to the inverter.
 2. The system of claim 1, wherein thepower-generation source coupled to the power module comprises aphotovoltaic power source.
 3. The system of claim 1, wherein the powermodule comprises a direct current to direct current (DC/DC) converter.4. The system of claim 3, wherein the power module, when disabled, isconfigured to cease conversion of input power from the input terminalsof the DC/DC converter to the converted power on the output terminals ofthe DC/DC converter.
 5. The system of claim 1, further comprising abypass route configured to carry current across the power module,wherein the controller is configured to activate the bypass route todisable the power module.
 6. The system of claim 1, wherein theelectrical parameter being out of the predetermined specification isindicative of an islanding condition.
 7. A method comprising: monitoringoperation of an inverter comprising an input coupled to one or morepower modules and an output coupled to a load; opening, in response toone or more electrical parameters of the load being out of apredetermined specification, a switch disposed in series between outputterminals of the one or more power modules and the inverter; detecting,in response to the switch opening, a change in current flowing throughan output terminal of the output terminals of the one or more powermodules; and disabling the one or more power modules in response todetecting the change in current, wherein the change in current comprisesthe current decreasing to zero, wherein the disabling comprises limitingpower to the input of the inverter from the one or more power modules.8. The method of claim 7, wherein disabling the one or more powermodules comprises stopping a supply of power to the inverter.
 9. Themethod of claim 7, wherein disabling the one or more power modulescomprises using a bypass route to bypass one or more terminals of theone or more power modules.
 10. The method of claim 7, further comprisingdisconnecting the inverter from an electrical grid.
 11. The method ofclaim 7, further comprising shutting down the inverter.